In a chip equipped with a digital circuit and a DAC (digital to analogue converter), a ground VSSC of a digital unit and a ground VSSA of the DAC are normally connected to a board with bonding wires. The bonding wire contains inductance components, so that voltages applied on both ends of the bonding wire vary every time AC currents flow. For example, currents flowing through the digital circuit during an operation of the digital unit flow in the inductance of the bonding wires, which causes variation in voltage of the VSSC. This variation in voltage is propagated through a back-to-back diode in the chip to the ground VSSA of the DAC.
One of problems in the related art is that, in a current steering DAC, variation in voltage (noises) propagated to the DAC is not cancelled, and thus noise components are contained in output analogue signals.
To address this problem, the following conventional solution is well known: a dummy path is provided along with a main path. In the case where noises of the ground are propagated through an emitter follower in the main path, so that the noises are applied to output signals, an emitter follower is also provided in the dummy path, and the same noise currents as those in the main path are generated. The respective differential output currents of the main path and the dummy path are combined each other with their polarities inverted. Consequently, the noise currents in the main path and the noise currents in the dummy path cancel each other due a differential relation, thereby reducing the noises.
In this conventional solution, matching between the main path and the dummy path is a crucial issue. Hence, there occurs a problem to reduce the variation in elements as small as possible. Mismatching between the parasitic capacitance value Cdp included in the main path and the parasitic capacitance value C′dp included in the dummy path may reduce noise cancelling effect.